The development of colloidal goethite from partially neutralized iron(III) nitrate solutions has been investigated by high resolution electronmicroscopy and ultracentrifuge analysis. Monocrystalline rod-like particles characterized by (001) faces and very flat (010) and (120) faces are observed to form. Within these crystals, a substructure of oriented bundles of needles packed in the  direction are also identified. The needles have a typical diameter of 3 nm similar to that of the aged amorphous particle which first forms from the supersaturated solution.\ud \ud It's shown that the needles can form both by oriented coagulation followed by recrystallization and by growth of an amorphous particle in the  direction through uptake of low molecular species from the solution.\ud \ud The mechanism by which the rod-like particles form from the needles is not clear, but probably does not involve oriented coagulation of the acicular cyrstals. The final size of the goethite particles is determined by the initial supersaturation of the solution with respect to the amorphous phase. Ionic strength has a large influence on the sol stability
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